Embodiments according to the present invention relate to methods and systems for determining a time difference (TDOA—Time Difference of Arrival) between a first time of arrival (TOA—Time of Arrival) of a signal and a second time of arrival (TOA) of the signal, and TDOA-based methods and systems for finding a position of the transmitter transmitting the signal.
The importance of radio direction-finding (DF) or position-finding (PF) technologies for navigation purposes is decreasing due to the omni-present availability of satellite navigation systems, for example GPS (Global Positioning System). Simultaneously, with an increasing mobility of communication means, the necessity increases to determine the positions of such communication means or any other transmitters or emitters. There is a series of fields of applications for direction-finding or position-finding technologies: (a) radio monitoring according to the guidelines of the ITU (International Telecommunications Union) for position determination of non-authorized transmitters or for the search of interference signals; (b) military reconnaissance, for example, for detecting activities of potential enemies or determining their position; and (c) security services, for example, for detecting radio communication of criminal organizations.
Today, a plurality of different direction- or position-finding methods exist. A good direction- or position-finding method is characterized by fulfilling as many of the following requirements as possible (see Carl-Cranz-Gesellschaft e.V.: Funkortung, Funkerfassung, Seminar FA 1.05, Oberpfaffenhofen, near Munich, 27-30 May 2008, in the following referred to as [6]: 1. high accuracy, 2. great sensitivity, 3. sufficient large signal strength, 4. immunity against distortions of the received fields due to multi-path propagation, 5. immunity against polarization rotations, 6. determination of the elevation in the short-wave range, 7. stable performance at non-coherent co-channel interferers, 8. short admissible signal duration, 9. high search speed and detection probability, 10. low costs, 11. simple operability, and 12. high flexibility.
There is no direction- or position-finding method, which combines all requirements in an optimum symbioses. In the following, as an example, some advantages and disadvantages of conventional direction- or position-finding methods are described.
The direction-finding with a directional antenna has the advantages of: high sensitivity due to antenna gain; low realization effort, only one receive train is necessitated; resolution of multi-wave fields possible; and direction finding and monitoring possible with the same antenna. The disadvantages are: detection probability reciprocal to directivity and rotating speed and only limited application for short-term signals.
Direction-finding based on the Watson-Watt principle has the advantages of: short signal duration sufficient, simple realization, and low space requirement. The disadvantages are: errors at multi-path propagation, and high direction-finding errors with sky-wave reception at steep elevation angles.
Doppler direction finders have the advantages of: high immunity against multi-path reception, and high sensitivity. The disadvantages are: high time requirements for gaining a direction-finding value and limited suitability for short-time signals in the VHF (Very High Frequency) and the UHF (Ultra High Frequency) range.
The direction-finding using correlation interferometers has the advantages of: high accuracy and sensitivity, polarization proof, and easy to be realized with digital signal processing. The disadvantages are: great space requirements for short waves.
A further method for determining a position of an object is hyperbola positioning, also referred to as multi-lateration positioning. The location of the target object is determined by computing the time difference of arrival (TDOA) of a signal emitted from that object to three or more receivers. As the distance between the object and the receivers is proportional to the propagation time of the signal transmitted from the object to the transceivers, the difference of the individual arrival times at the receivers can be used to determine, for each receiver pair, a respective time difference for the arrival or reception times, which again can be used to determine a hyperbola that represents possible locations of the transmitter with regard to the receivers of the corresponding receiver pair. By using at least two or more receiver pairs and their corresponding hyperbolas, the position of the transmitter can be determined. A typical solution to calculate the time difference for a receiver pair is to use a correlation of the signals received at the receivers of the receiver pair. The temporal resolution for a conventional correlation, however, is limited to a sample period. As the accuracy of the TDOA based position determination increases, e.g. with the temporal resolution of the correlation, typically interpolation filters are used to obtain finer temporal resolutions of the time difference in case the sample period is limited. However, the choice of the interpolation filter influences the calculation of the time difference so that depending on the choice of the interpolation filters, different time differences and different hyperbolas are obtained.
Therefore, there is a need to provide an efficient and accurate possibility to determine the time difference of signals originating from the same source to perform a position detection of the source.
However, the need for efficiently determining an accurate time difference of two representations of the same signal is not only limited to applications like position detection, but may also be used in other areas, where a difference of reception times of two representations of a signal at two different receivers or at the same receiver needs to be determined.